92 Disease and Immunity 



weak one produced by a long period of idleness 

 in life on artificial food, and one not so weak 

 produced by a shorter period of idleness. He then 

 inoculated an ox with the weaker culture, and 

 twelve days later with the stronger culture. An 

 animal so inoculated was immune to fully 

 virulent virus. Here we have the progressive 

 decay of powers in the bacilli by idleness, and the 

 progressive development of powers in the ox by 

 exercise. 



But Pasteur's experiment went still further 

 in this matter. By long cultivation on artificial 

 food he got anthrax germs so weak from the 

 lack of exercise in fighting for food that they 

 were unable to survive even in a mouse. But 

 by taking such weak virus and inoculating a 

 very feeble animal, as a guinea pig a day old, 

 and then passing it along by inoculation to 

 stronger and stronger animals, he found that the 

 strength of the virus was built up step by step 

 with each inoculation until it was powerful 

 enough to attack the strongest animals. Here 

 we have a case of absolute control over the gain 

 and loss of powers in the same organism by 

 controlling the amount of its exercise. 



